1. Field of the Invention
The present invention relates to a ballast apparatus and ballasting method of a high intensity discharge lamp, such as a metal halide lamp, used as the light source of the headlights of an automobile or the like.
2. Description of Related Art
Recently, metal halide lamps have been used as the headlights of an automobile instead of halogen lamps. The metal halide lamp, one of the HID (High Intensity Discharge) lamp, is characterized by its high luminous efficiency, high color temperature and long life compared with the halogen lamp.
The arc tube of the metal halide lamp contains metal halides which are mixtures of some metals such as sodium and scandium with halogen such as iodine, high pressure xenon serving as a starting gas, and mercury. The metal halide lamp starts emitting light as follows. First, it starts discharge of the xenon in a gaseous condition at room temperature, followed by arc discharge of the xenon, which increases the temperature inside the arc tube. As the temperature in the tube increases, the mercury vaporizes and starts arc discharge, thereby further increasing the temperature in the tube. A further increase in the temperature in the tube brings about the evaporation of the metal halides, followed by their arc discharge, thereby achieving the high color temperature emission at high luminous efficiency. Incidentally, although the mercury serves as a stopgap of the discharge between the xenon and metal halides, metal halide lamps without containing the mercury have been provided recently.
Thus, it is necessary for the ballast apparatus of the metal halide lamp to sequentially discharge the variety of the substances to keep the discharge. In other words, it must control the discharge in response to the variable load characteristics of the lamp.
Consequently, the ballast circuit of the metal halide lamp must meet singular requirements which differ greatly from the requirements for the ballast circuit of fluorescent lamps (low-pressure mercury vaporization discharge lamps) widely used as normal household lighting and backlights of liquid crystal displays.
As the ballast apparatus of vehicle headlights using the metal halide lamps, which satisfies the foregoing requirements, the technique disclosed in Relevant Reference 1 is known. In the present specification, the circuit configuration described in Relevant Reference 1 is called “full-bridge, low-frequency ballast system”.
Although the full-bridge, low-frequency ballast system can meet the requirements necessary for the metal halide lamp and implement a rather compact and inexpensive ballast apparatus, further reduction in the size and cost is required.
On the other hand, as for the ballast apparatuses of fluorescent lamps used as the backlights of liquid-crystal displays, a thoroughgoing reduction of their size and cost has been carried out. As a result, the methods have been widely used which convert voltages fed from DC power supplies to AC waves using push-pull DC-AC inverters to light the lamps at high frequencies. As an example of such methods, a ballast circuit described in Relevant Reference 2 is known. In the present specification, the circuit configuration disclosed in the Relevant Reference 2 is called “DC-AC one-step-boost high frequency ballast system” from its circuit characteristics.
The DC-AC one-step-boost high frequency ballast system carries out the power conversion from DC to AC only once within the ballast circuit using a push-pull DC-AC inverter. Thus, it can simplify the circuit configuration, and miniaturize its transformer occupying a large portion of the total volume of the apparatus because of the high frequency turn-on, thereby being able to achieve the reduction in size and cost of the ballast apparatus. To apply the circuit scheme to the ballast apparatus of the metal halide lamp, the singular requirements of the metal halide lamp must be met, which prevents the implementation thereof.
Relevant Reference 1: Japanese patent application laid-open No. 2002-352989.
Relevant Reference 2: Japanese patent application laid-open No. 7-211472/1995.
The full-bridge, low-frequency ballast system described above has its discharge developing capacitor connected in parallel with the secondary output of the transformer for generating a high voltage. Accordingly, it requires a capacitor with a high withstand voltage, hindering the reduction in cost and size of the high intensity discharge lamp ballast apparatus.
In addition, the full-bridge, low-frequency ballast system separates its DC-DC converter for boosting from the inverter for converting into AC to satisfy both the requirements to reduce the size of the transformer and to stabilize the lamp. Thus, it requires the multi-stage conversion, which increases the number of the circuit components, and prevents the reduction in size and cost of the high intensity discharge lamp ballast apparatus.
Furthermore, to satisfy the requirements to start the metal halide lamp, it is necessary to generate a high voltage pulse of about 20 kV or more across the lamp to cause breakdown in a hot condition during a discharge start period. However, it is necessary for the circuit using the DC-AC one-step-boost high frequency ballast system to increase the number of turns of the inverter transformer to boost to the high voltage. This increases the size of the transformer, hampering the reduction in size and cost of the high intensity discharge lamp ballast apparatus.
Moreover, the circuit using the DC-AC one-step-boost high frequency ballast system employs the same frequency for driving the transformer and for turn-on the lamp. As a result, to turn on the lamp at a low-frequency to stabilize the discharge, the size of the transformer increases, which prevents the reduction in size and cost of the high intensity discharge lamp ballast apparatus.